1. Field of the Invention
The invention relates to an electrical circuit with a thyristor for switching frequencies greater than 10 kHz and forward blocking voltages greater than 500 V, in which the thyristor has an interdigitated emitter-gate structure, a control zone in contact with the gate, and a base zone which is less doped than the control zone. When the thyristor is being turned off, the gate remains at a potential whose polarity is opposite to that of the majority carriers in the control zone, and the control zone does not have any emitter shorts which might short-circuit the PN-junction with the adjacent emitter zone, to the electrode contacting the emitter zone. Further, the invention relates to a method for driving the above-described circuit arrangement.
2. Description of the Prior Art
A circuit arrangement of the type described above is disclosed in Int. J. Electronics 36 (1974), pp. 399-416. Circuits in which such electrical connections can be used are known, for example from Heumann, Stumpe "Thyristors," Teubner-Verlag 1970, pp. 131-133, or Penkowski, Pruzinsky "Fundamentals of a pulse Width Modulated Power Circuit" Power Semiconductor Applications, IEEE Press 1972, pp. 266-275, or "Silicon Rectifier Handbook," Aktiengesellschaft Brown, Boveri & Cie., Baden (Switzerland), 1971, pp. 197-201.
The thyristor disclosed in Int. J. Electronics 36 (1974), pp. 399-416, has recovery (turn-off) times of less than 2 .mu.sec, a permissible voltage gradient du/dt=600 V/.mu.sec, a forward and reverse blocking voltage of 650 V, and a switching frequency of 100 kHz.
The low recovery (turn-off) time is substantially attained through the above-described control of the gate. The negative (in the case of a P-control zone) potential at the gate causes the charge carriers of the ON-condition, which is to be switched to the OFF-condition, to be cleared from the control- and base-zones more quickly. This principle is also known as "gate-assisted turn-off" (GATO). However, this principle entails the danger that, in the case of larger diameters of the semiconductor component to be switched to the OFF state, the current may be "crowded" into a channel of small diameter, and cause overheating and damage there. In order to prevent this, the interdigitated emitter-gate structure is provided, by means of which the negatively polarized gate can rapidly act on all parts of the P-control zone. A high conductivity of the P-control zone is also of assistance in this context. Emitter shorts which short circuit the PN-junction between the control zone and emitter zone on the cathode side (in the case of a P-control zone), should not be provided in this case, as otherwise the negatively polarized gate would be without effect. Indeed, the P-N junction must have a sufficiently good blocking ability, as this ability restricts the possible negative potential at the gate.
The interdigitated emitter-gate structure is not only useful for turning the thyristor off, but is also of great importance for turning it to the ON state, as only in this way can firing, proceeding from the gate, sufficiently quickly reach all the active parts of the thyristor. Otherwise there would be unacceptably high switching losses and too low permissible current rise times.
In this connection it is true that appreciable limitations are imposed due to the requirement for high firing currents for the purpose of uniformly firing the long cathode edge, and also to the need for very small cathode strip widths. If the whole cathode surface is to be fired within one .mu.sec, firing currents of the order of magnitude of the forward current in the ON state of the thyristor would be needed, as would cathode strip widths of only about 300 .mu.m. However, this would entail the loss of so much active surface that this expedient does not appear to be suitable in practice.
The electrical circuit layout disclosed in Int. J. Electronics 36 (1974) pp. 399-416, is also not suitable for higher blocking voltages, for example, those over 1000 V. This is because high blocking voltages require a thick, high-ohmic base zone in the thyristor. However, such a base zone prevents sufficiently rapid clearing of the charge carriers from the ON state, if--through the incorporation of recombination centres (for example, gold doping)--the lifetime of the charge carriers is not to be appreciably shortened. However, again, such recombination centres cause an unacceptably high (thyristor) ON state voltage drop.